As a wet developing system in electrophotography, a process is generally known which comprises giving charges and image exposure on a photosensitive member to form electrostatic latent images, developing the images with a liquid developer comprising, for example, an aliphatic hydrocarbon carrier and toner particles dispersed therein which contains a resin and a colorant as main components, transferring the resulting toner images to a transfer paper sheet, and fixing them, thereby forming images. FIG. 1 shows a schematic construction of an example of an apparatus for conducting such a process. Referring to FIG. 1, a photosensitive member 101 is uniformly charged by use of charging corotrons 102, and an exposure device 103 provides image exposure thereon to form latent images, followed by contact with a liquid developer contained in a developing unit 104, thereby developing the latent images. The toner images thus formed are then transferred to a transfer paper sheet 106 with a transfer corotron 107, and fixed with fixing unit 108. Charges and toner particles remaining on the photosensitive member 101 are removed by means of an eraser lamp 109, a cleaning roll 110 and a cleaning blade 111, making preparations for the next cycle. The reference numeral 105 indicates a metering roll.
When photosensitive paper or a photosensitive film coated with a photoconductive material such as zinc oxide or titanium oxide is used as the photosensitive member, the transfer procedure in the above-described process may be omitted to directly fix the formed toner images on the photosensitive member after development.
Also when a liquid developer is used as a developing means in an electrostatic recording system in which electrostatic latent images are formed on a dielectric member by electric input without using the photosensitive member, the latent images are transferred and fixed in a manner similar to the above.
In a wet developing system, fine toner particles having a size of submicrons to several microns are dispersed in a carrier liquid having a high electric resistance such as an aliphatic hydrocarbon compound, and electrostatic latent images are developed mainly based on the principle of electrophoresis. This system therefore has the feature that high resolution image quality is easily obtained rather than in a dry developing system in which toner particles having a size of more than several microns are used. Two early literatures introduced by K. A. Metcalfe (J. Sci. Instrum., vol. 32, 74 (1955), and ibid., vol. 33, 194 (1956)) disclose that a number of organic and inorganic pigments, including carbon black and magnesium oxide, can be used as pigments (toners) in liquid developers, and that gasoline, kerosine, carbon tetrachloride, etc. can be used as carrier liquids.
Metcalfe's early patents also disclose that halogen-containing hydrocarbons (as described in JP-B-35-5511, the term "JP-B" as used herein means as "examined Japanese patent publication), polysiloxanes (as described in JP-B-36-14872), and ligroin and mixtures thereof with petroleum hydrocarbons (as described in JP-B-38-22343 and JP-B-43-13519) can be used as carrier liquids. Many descriptions of carrier liquids are found in patents relating to toner producing methods (as described, e.g., in JP-B-40-19186, JP-B-45-14545 and JP-B-56-9189). These literatures describe that aromatic hydrocarbons such as toluene, xylene and benzene, and aliphatic hydrocarbons such as esters, alcohols, n-hexane, isododecane and Isopar H, G, L and V manufactured by Exxon Chemical Co. can be used as carrier liquids which sometimes act as dispersing media for polymerization of toner particles. However, these carrier liquids previously proposed are organic solvents having a high vapor pressure, and therefore have the problems in that: (i) vapor of the carrier liquids discharged on fixing is liable to cause environmental pollution, and (ii) the carrier liquids are flammable.
In order to solve these problems, for example, it is proposed to use hydrocarbon-series petroleum solvents that are low in vapor pressure for preventing vapor of the carrier liquids from being generated. However, an increase in the molecular weight of the hydrocarbons for the purpose of lowering the vapor pressure generally causes an increase in the viscosity of the carrier liquids, which results in an adverse effect on the rate of development. Further, the melting point of the carrier liquids is elevated to near room temperature, which requires continuous heating for using them in the liquid developers. This is unfavorable in terms of energy saving, heat pollution and deterioration of the developers.
Furthermore, the use of the carrier liquids having a low vapor pressure in the liquid developers causes penetration of excess carrier liquids into transfer paper. Accordingly, the problems of image quality arise that non-image parts become transparent and that stains due to the carrier liquids are liable to be produced.
In order to solve these problems, a liquid developer is proposed in which an oil coagulating agent is added to the insides of toner particles to adsorb a carrier liquid by heat on fixing (as described in JP-A-62-49363, the term "JP-A" as used herein means an "unexamined published Japanese patent application). In this case, however, the carrier liquid can only be absorbed in the vicinity of toner images, and can not be satisfactorily absorbed over the whole region on a transfer paper sheet. Furthermore, the oil coagulating agent described in JP-A-62-49363 does not generally dissolve in a toner binder, resulting in a decrease in the strength of fixed toner images or exertion of an adverse effect on toner electrification. In addition, the problem is also encountered that the resolution is decreased by absorption of the carrier liquid. Thus, satisfactory results have not been obtained yet.
As described above, with respect to the carrier liquids for liquid developers previously proposed, no means which can fully meet recent environmental regulations, or which fundamentally reduces the amount of carrier liquids discharged from copying machines or printers using liquid developers has been obtained yet.